Soil Physical and Chemical Characteristics Influencing Hydrogen Sulfide Toxicity

Hydrogen sulfide (H₂S) toxicity is a poorly understood physiological disorder that occurs under anaerobic conditions and can cause substantial yield loss in rice (Oryza sativa L.). Though the reduction of sulfate (SO₄^2-) to H₂S is the causes of toxicity, there are many factors that influence the extent to which this occurs. Two greenhouse studies were designed to investigate the chemical and physical characteristics of four soils in Arkansas where this disorder occurs regularly (H and HR-W), sometimes occurs (HR-E), and has never been reported (PTRS). The three soils that have had this disorder (H, HR-W, and HR-E) contained approximately 30% more silt than PTRS. Mehlich 3 extractable SO₄^2- and Fe concentrations were significantly different among the soils. In the first study, the effect of soil sterilization on SO₄^2- concentration was examined. This study showed that SO₄^2- concentrations over time were significantly greater in the sterilized soils from day 7-77 (p=0.0231 to <0.0001) indicating that microbes play a key role in the disappearance of SO₄^2-. Sulfate concentrations were different from day 21-77 (p=0.0310 to <0.0001), however H and PTRS were not statistically different. Redox potential dropped more rapidly in H than PTRS, suggesting that redox potential greatly influences the occurrence of H₂S toxicity. When rice was grown, there was again a statistical difference between locations (p=0.0405 to 0.0095), however H contained the most SO₄^2- and PTRS the least. The most rapid decline in SO₄^2- occurred after two weeks of flooding, which coincides with the onset of symptoms in the field. Within four weeks after flooding, H lost 20.7 mg SO₄^2- kg^-1 soil solution whereas PTRS lost 13.5 SO₄^2- kg^-1 soil solutions. These results indicate that the rate of SO₄^2- reduction, decline in redox potential, and activity of microorganisms all play a role in the occurrence of H₂S toxicity.